Purpose: :
Mean ocular refraction (MOR) decreases with age inthe chick eye by an active process known as emmetropization.We wished to investigate the rates of change of the opticaldimensions of the chick eye during the first 14 days after hatching.We compare the predictions of passive and active models of growthbased on these changes with measured MOR and image quality inorder to determine underlying changes in blur sensitivity.

Methods: :
We have previously reported an improvement in opticalquality across both constant and growing pupils in the naturallydeveloping chick eye. We refit data provided by Irving and colleagueson age changes in ocular parameters with significant exponentialregressions. The rates of change of pupil radius, anterior cornearadius and ocular equatorial diameter were used to give magnificationfactors for scaled eye models. We also developed active models.In the first, the retinal position maintained a constant defocusblur. In the second, a constant linear blur due to higher–orderaberrations was maintained with increasing eye size. Expectedvariations in MOR and aberrations with age were derived foreach model.

Results: :
The pupil radius rate of increase is not significantlydifferent from those of any of the other anterior ocular parameters.Our models of passive growth increase in size exponentiallywith age. The passive model predicts a decrease in MOR as theinverse magnification. For constant retinal blur, MOR decreasesas the inverse square of the magnification. The rate of decreaseof measured MOR was more rapid, implying an exponential improvementin sensitivity to defocus blur with a time constant rangingbetween 5.6 & 15.2 days. The passive model predicts an increasein aberrations with age. The active model predicts that theroot–mean–square wavefront aberration remains constantand the angular blur on the retina, approximated by equivalentblur (Kisilak et. al., 2005), decreases as the inverse magnification.The experimentally observed improvement in equivalent blur inthe developing chick eye implies an improvement in sensitivityto aberration blur, significantly smaller than the rate of changein sensitivity to defocus blur.

Conclusions: :
Measured changes in defocus and aberration blursare not predicted by passive models of eye growth. These resultssuggest an improvement in sensitivity to defocus blur with age.The data are consistent with two different mechanisms of defocusand aberration control and two different blur sensitivities.